The nano-scale viscoelasticity using atomic force microscopy in liquid environment

Area of Science:

• Nanoscale science
• Biophysics
• Physical chemistry

Background:

• Quantifying viscoelasticity in nanoscale systems is crucial for understanding molecular behavior.
• Previous studies reported significant dissipation in single protein molecules, but experimental limitations may have contributed to these findings.
• Atomic force microscopy (AFM) is a key technique for probing mechanical properties at the nanoscale.

Purpose of the Study:

• To accurately measure the viscoelasticity of single protein molecules and confined water layers.
• To investigate and identify sources of artifacts in AFM-based dissipation measurements.
• To compare the performance of different AFM detection schemes for viscoelasticity measurements.

Main Methods:

• Utilized two AFM detection schemes: commercial deflection detection (cantilever bending) and fiber-interferometer based detection (cantilever displacement).
• Modeled cantilever hydrodynamics using the Euler-Bernoulli equation to account for different detection methods.
• Performed measurements on single octomers of titin (I27 b8) and molecular layers of water confined between solid surfaces.

Main Results:

• The dissipation coefficient of single titin I27 b8 was found to be immeasurably low (upper bound 5 × 10 b-7 kg s b-1), contradicting literature values.
• Entropic stiffness of single unfolded protein domains was measured at approximately 10 mN m b-1 using both methods.
• Identified phase artifacts in conventional deflection detection as a major source of error in dissipation estimates.

Conclusions:

• Cantilever displacement measurement offers superior accuracy for artifact-free viscoelasticity measurements compared to bending detection, especially at low frequencies.
• The low dissipation observed in single protein molecules suggests a need to re-evaluate their role in energy dissipation mechanisms.
• Findings provide insights into discrepancies in protein collapse dynamics observed between force spectroscopy and optical techniques.